The invention relates to a method for monitoring a secondary air system in combination with the exhaust-gas system of a motor vehicle.
A method of this kind and an arrangement for carrying out the method are known from U.S. Pat. No. 5,542,292. With this known method, the operability of the secondary air system can be checked but it is not possible to determine the secondary air mass actually supplied to the exhaust-gas system. All emission relevant components and systems of controls for internal combustion engines must be able to be diagnosed in the context of the on-board diagnosis (OBD) required by the American authorities and which is intended to also take effect in Europe starting Jan. 1, 2000. Here, the quantitative determination of the actual secondary air mass is also required.
The invention is based on the task of further improving a method for monitoring a secondary air system in combination with the exhaust-gas system of a motor vehicle of the generic type in such a manner that the secondary air mass, which is actually supplied to the exhaust-gas system, can be determined without an additional sensor, for example, an air mass sensor or the like, being required.
In the method according to the invention, the operability of the secondary air mass system is evaluated on the basis of the arithmetically determined secondary air mass flow. An advantage of the invention is that the secondary air mass can be determined without an additional sensor because the signals, which are necessary for the determination of the secondary air mass, are either anyway available or can be formed from signals which are anyway available.
The secondary air mass can be determined with a continuous xcex-probe as well as with a two-point xcex-probe.
A valve tightness check of the secondary air mass cutoff valve and an offset measurement to determine the secondary air mass offset when the secondary air pump is switched off are the subject matter of advantageous additional embodiments of the method.
The diagnosis takes place in a pregiven air mass window and engine temperature window in order to exclude malfunctions during the measurement phases. Transient influences are substantially eliminated by suppression and a lowpass filtering of the measurement results.